Tuned circuits



Jan. 13, N42. J. D. BRAILSVFORD TUNED CIRCUITS Filed July 29, 19:59

4 Sheets-Sheet 1 INVENTOR JOSEPH D.BRA/L FORD ATTO R N EY Jan. 13, 1942. J. D. BRAILSFORD 2 TUNED CIRCUITS Filed July 29, 1959 4 Sheets-Sheet 2 -INVNTOR JOSEPH D. BRA/FORD ATTORNEY Jan. 13, 1942. J. D. BRATLSFORD 2,270,017

TUNED CIRCUITS Filed Jul 29, '1959 4 Shets-Sheet :s

100-0. AERIAL FEEDER INVENTOR ATTORNEY n. 13, 1942 .1. D. BRAILSFQRD 2,270,011

TUNED CIRCUITS I Filed July 29, 1939 4 Sheets-Sheet 4 7 l- 000.5'MAX.

INVENTOR JflS'Pl-I 0. BR% $FORD we/p ATTORNEY Patented Jan. 13, 1942 TUNED CIRCUITS Joseph Douglas Brailsford, London, England, assignor to Radio Corporation of America, a corporation of Delaware Application July 29, 1939, Serial No. 287,277 In Great Britain September 5, 1938 6 Claims. (Cl. 250-) The present invention relates to tuned elec trical circuits and has for its object to provide improved tunable pre-selector circuits of high and substantially constant selectivity and suitable for use in radio and like receivers. e

The invention finds particular use in a super heterodyne receiver though it is not limited to such a receiver.

In superheterodyne radio receivers pre-selectionis, in practice, necessary in order to avoid.

or reduce whistling, image signals, noise and are of little moment compared with direct inter-g;

ference, if the I. F. selectivity is high cross modu lation and demodulation efiects occurring when it is sought to receive a weak signal adjacent to a strong signal become the principal cause of interference. modulation effects can be reduced by increasing the selectivity of the pre-selector circuits of the receiver and it is the principal object of the present invention to provide, with this object in view,

improved signal frequency circuits of high selec- Such cross modulation and detivity and of substantially constant selectivity with varying frequencies which shall be simple and inexpensive.

According to 'the present invention a tuned radio frequency circuit or filter comprises one 013;;

more networks whereby as tuning or resonant frequency is reduced the effective resistance is increased so as to maintain the selectivity substantially constant over the tuning range.

In carrying out the invention the ratio Q on coil inductance to coil resistance in the tuned circuit is manitained constant so that selectivity shall be independent of frequency.

For the same degree of selectivity at signal frequency as at intermediate frequency the Q ofl j the signal frequency coils must be equal to the Q of the intermediate frequency coils multiplied by the ratio of the signal frequency to the intermediate frequency. Moreover this proportionality must be maintained throughout the tuning range.

The requirements for the commonly used band pass transformer for an intermediate frequency of 450 kc./sec. have been established. In order to get the same performance at signal frequency as that given by four circuits at intermediate frequency, four circuits would be required each having a Q of v at 1500 kc./sec. and a Q of 100-33 at 150 kc./sec., these frequencies representing the higher limit of the medium wave broadcast band and the lower limit of the long wave band respectively. The Q value must be made to be proportional to frequency within these limits.

By providing a Q of 330 at a frequency of 1500 kc./sec. the Q of the circuit can be maintained at the correct value at any other frequency simply by increasing the effective resistance of the circuit as the tuning frequency is decreased.

It would obviously be impracticable to obtain a high enough Q for Q to be made proportional to frequency right from the short wave bands down to the lowest frequency. The highfrequency therefore which need be considered is the highest frequency in the medium wave band. This frequency is normally set at about 1500 kc./sec. but except for local areas where special precautions could be taken, there are not many stations of great importance above 1200 kc./sec. so that the circuits need be designed only to have the highest possible Q at 1200 kc./sec.

The design of coils of the required Q for the highest frequency to be received follows well established principles and by following these principles a Q of 300 can be obtained at 1200 kc./sec. with a coil of 260 microhenries. Such a coil will provide a working Q of 260.

The making of Q proportional to frequency may be accomplished by including in the tunable circuit a suitable network which throws resistance thereinto according to the correct law.

The invention will be further described in connection with the accompanying drawings where- Figures 1 and 2 will serve to explain certain aspects of the invention, the former being a simple tuned circuit and the latter representing curves which show the relationship between certain resistance values and the frequency.

Figures 3a and 3b are networks which have the resistance-frequency characteristic shown by the curve T2 in Figure 2. Figure 3c is an equivalent circuit.

Figures 4a and 4b are networks equivalent respectively to Figures 3a and 3b and are utilized when it is desired that the resistance R be fixed.

Figure 5 shows a correction circuit applied to each of a pair of circuits constituting the bandpass selector or filter in accordance with the invention; and Figure 6 shows the correction circuit applied to only one of the filter circuits.

Figure '7 shows an elementary matching circuit interposed between an aerial feeder system and the band-pass preselector; and Figures 8a, 8b, 8c are various equivalent practical forms of the schematic circuit shown in Figure '7.

Figures 9, l and 11 are complete circuit diagrams of several improved band-pass tuners according to the invention.

Figure 12 shows the response curves for the system of Figure 11 measured at the three indicated frequencies in the tuning band, and

Figure 13 shows a circuit arrangement according to the invention utilizing the correction circuit of Figure 3b.

Referring now to Figure l the simple tuned circuit comprises the inductance L, the parallel tuning condenser C, and rl represents the total lumped high frequency resistance in the circuit, the circuit here represented not being in accordance with the invention. In this figure, resistance rl can be measured by means of a high frequency bridge and the relationship between effective resistance 1'1 and frequency plotted. This relationship is represented by the curve rl in Figure 2, resistance r (ordinate) being plotted against frequency (abscissa).

Now Q=wL/T1, therefore if Q is to be proportional to frequency, L/Tl must remain constant and, if L is fixed and Mn is to be constant, r1 must also remain constant. Curve r1 shows the required curve. tween curve T1 and curve n and represents the resistance which must be added to the circuit to maintain the total effective resistance constant. The limits A and B represent the limits of the tuning band concerned. Thus a network is required which will have the resistance/frequency law shown for 1-2. This network may be added to the circuit of Figure 1 where indicated at Z.

Figures 3a and 3b show two networks which have nearly the required law. These networks are equivalent to a small resistance r in series with a reactance X as shown in Figure 3a. In practical circuits the effect of the added reactance X can be neglected.

The values of the elements in Figures 3a or 3b can be derived by calculation such that the resistance T1 of Figure 2 has the required value at three (or other desired number of) points in the tuning range. The deviation from the required curve at points within the band at points other than these three is then small.

As it is sometimes inconvenient for resistance R in the correction circuit to be one of the variables, equivalent networks, in which R can be fixed, as shown in Figure 4a and Figure 4?) may be used.

The values shown in Figures 3a and 3b and 4a and 41) for four equivalent networks and found in the above manner, are suitable for correcting an actual coil of 260 microhenries inductance and low loss.

In the case of a band pass tuner employing two similar circuits, in which, as is required, Q is proportional to frequency, the ratio of coupling reactance to coil resistance must remain Curve T2 is the difference beconstant if the band width is to remain constant. When the filter consists of fixed inductances tuned by variable condensers mixed coupling can be used to give a sufficiently close approximation to this condition.

Figure shows how the invention may be applied to a band pass tuner by adding a correction circuit to each of the tuned circuits comprising the filter, and then proceeding with the design in the normal way, but, of course, bearing in mind that the coil resistance now remains constant for varying frequency. In Figure 5, Z1 and Z2 are the correction networks.

If the coupling reactance of the circuit shown in Figure 5 be X and the coil constant resistance be r, the coupling factor 5 is equal to X/r. When Q is proportional to frequency, the shape of the response curve is determined entirely by ,8 and therefore for constant band width and constant selectivity ,3 must remain constant. Since r is constant X must remain constant. If the coupling reactance consists of a negative mutual inductance in series with a capacity, the magnitude of the coupling reactance is given by the expression It is evident that this expression is not independent of frequency but it can be made to have the correct value at two frequencies within the band and by properly choosing these values the variation over the rest of the band may be of negligible consequence.

So far the band pass circuit has been considered as a symmetrical circuit employing coils of identical high frequency resistance.

The correction networks for making Q proportional to frequency add some expense and so the possibility of applying the correction to one half only of the band pass transformer has been investigated, and it has been found possible to effect this by suitably modifying the dimensions of the coupling elements. Where, for economic reasons, it is desirable to apply the correction to one circuit only, the symmetry of the circuit is upset. This involves consideration of a band pass tuner having coils of different Q.

If n be the series H. F. resistance of the first circuit and m the series H. F. resistance of the second circuit and their ratio be defined by the parameter m, where then:

2K =2p (m +1) (1) where .60: K=

j is the frequency to which the filter is tuned, and A is the frequency oil-tune at which the maxima of the double-humped curve occur (the coupling is assumed to be greater than critical coupling). Q here refers to the first circuit so that if we apply the correction to this circuit, Q=kf where k is some constant. Thus K=27cAf.

But 2A is the peak separation of the doublehumped response curve, so for constant band width we can assume 2Afa constant, therefore K becomes a constant which can be determined. m which is the ratio of the measured high fre- 75 quency resistance of the second circuit to the automatically maintained constant resistance of the first circuit is therefore also known.

Substituting the values of K and m in Equation 1 the law of against frequency required to maintain constant response can be determined.

Mixed coupling consistingjof a negative mutual inductance in series with a fixed condenser can again be made to give values of 5 which fulfill this law to a sufiicient degree of accuracy.

Figure 6, shows the general arrangement of such a circuit. Z1 may be, for example, as Figure 3a or 31) with the values marked therein. If the network of Figure 3b is used the correction circuit may be combined with the coupling elements.

Figure 13 shows an arrangement according to the invention and using the network of Figure 31) combined in this manner with the values marked thereon.

For constant efficiency the aerial should throw into the first circuit of the tuner a resistance which bears a fixed ratio to the total effective series resistance acting in the first circuit (taking into account all the meshes of the filter). As, of course, will be appreciated for maximum efficiency the resistance thrown in from the aerial should equal the circuit resistance. To throw in so large an amount of resistance would obviously spoil the characteristic of the filter. It is found that with very weak aerial coupling such that the Q of the first circuit was reduced by only 15% the loss in efficiency due to aerial mismatching is very small and results in only and if R. is small compared with (X1+X2)2 the expression may be written X (T+X; This gives a very convenient form of matching circuit which is independent of frequency. R. can be made 100 ohms and be constituted by a correctly matched aerial feeder system and X1 and X2 can be very simply calculated to give the required value of 15.

Figures 8a, 8b and 80 (which are for illustrative purposes only) show various equivalent practical forms of this fundamental circuit, capacity, inductive, and transformer coupling being used respectively.

The method of Figure 817 has considerable advantage over the more usual arrangement shown in Figure 80 when coils with completely closed iron dust cores are being used. In such coils it is often very difficult to provide a mutual coupling with a large enough value of leakage inductance, because the coupling coil or primary, cannot be put partially out of the field.

The aerial is represented as being coupled to the aerial was tapped down so low that the band pass coupling reactance represented 30% of the total reactance between the aerial tap and earth.

This factmakes it possible to arrange thecircuit so that the tuning condensers, the band pass coupling elements and the aerial feeder are all earthed on one side.

Figure 9 shows a complete circuit having high frequency resistance correction applied to the first circuit, mixed band pass coupling, and improved aerial coupling, according to the invention.

Finally instead of making the aerial throw a constant resistance into the circuit it is possible to make it throw in a resistance which varies with frequency and by employing either of the resistance networks of Figure 4 for the purpose of aerial coupling, the resistance thrown in by the aerial can be made to constitute the whole of the correcting resistance. Thus in Figure 4a or 41) the 100 ohms resistance is replaced by the 100 ohm aerial feeder, and the network, designed so that at 1200 kc./sec. it throws into the first circuit of the tuned circuit, the minimum amount of resistance to give efiicient aerial coupling with a mismatching loss not greater than 3 db.

An increasing amount of resistance will be thrown into the first, circuit from the aerial as coupling to the valve anode, is shown in Figure 11.

a simple circuit to demonstrate the equivalence In Figure 11 the small inductances at A are added to compensate for the band pass coupling elements included in series with the condensers B.

In both Figure 10 and Figure 11 the coils of the second circuit of the band pass tuner are shown as being tapped down, such a measure being adopted only where the valve grid to which the circuit is connected has a high di-electric loss or is liable to vary greatly in capacity for different valves of the same type.

In Figure 12 the full line curve relates to the band 600 kc./sec., the dash line to 800 kc./sec. and the dot and dash line to 1200 kc./sec. It will be seen (Figure 12) that the response curves which relate to the amplifier of Figure 11, and which were taken at the three points in the tuning band, 600 kc./sec., 800 kc./sec., and 1200 kc./sec., are practically identical.

I claim:

1. A band-pass preselector circuit comprising two tuned circuits, fixed coupling means between said circuits, and correcting circuit means having the inherent characteristic .of decreasing in resistance with increase in frequency included in at least one of said tuned circuits, whereby as the frequency of the circuits is reduced the effective resistance of said tuned circuit increases whereby to maintain the selectivity substantially constant over the tuning range, said correcting circuit means consisting of a network comprging inductance, capacit and resistance.

2. A band-pass preselector circuit as defined in claim 1 wherein the correcting circuit means is shunted across the coupling means.

3. A band-pass preselector circuit as defined in claim 1 wherein the correcting circuit means is connected in series with the tuned circuit elements.

4. A band-pass preselector circuit as defined in claim 1 wherein an aerial is coupled to the preselector circuit and the resistance of the aerial constitutes the resistance of the correcting net- Work.

5. A band-pass circuit comprising two tuned circuits adapted to be tuned over a predetermined frequency range, reactive means for coupling said circuits, and correction circuit means included in at least one of the tuned circuits adapted to maintain a substantially constant ratio of coupling reactance to coil resistance thereby to maintain a substantially constant band-width over said frequency range, said correction circuit means comprising an electrical network of inductance, capacity and resistance, the effective resistance of said network having the characteristic of decreasing with increase in frequency.

6. A band-pass circuit as defined in claim 5, wherein an aerial is coupled to the first circuit of the filter and is adapted to reflect into said circuit a resistance which varies with frequency.

JOSEPH DOUGLAS BRAILSFORD. 

